Production of anticorrosive protective coatings on light metals



Patented Aug. 22, 1933 UNITED STATES PRODUCTION OF ANTICORROSIVE PRO- TECTIVE COATINGS N LIGHT METALS Alexander Jenny, Berlin-Charlottenburg, Heinz Siebeneicher and Nicolai Budiloff, Berlin- Friedenau, Germany, assignors to Siemens- Electro-Asmose G. in. b. H., Siemensstadt near Berlin, Germany, a Corporation of Germany No Drawing. Application September 16, 1930, Serial No. 482,368, and in Germany September '7 Claims.

Our invention relates to improvements in the production of anticorrosive protective coatings on light metals, more particularly on aluminium and magnesium as well as their alloys. In contradistinction to the methods or processes of this kind already known in the art protective coatings of excellent quality on light metals are, according to our invcntion,-obtained by subjecting the articles to be coated in an aqueous electrolyte to the action of an alternating electric current. The articles of light metal serving as electrodes after some time become covered with a protective coating or film which prevents corrosion. The coatings or films obtained by our improved method differ according to the composition of the electrolyte, the chemical composition of the articles, their mode of manufacture and condition of surface as well as the voltage and density of the applied alternating current, the temperature of the electrolyte and the duration of the treatment.

The articles to be coated according to our improved method are first freed from all grease in the usual manner. They are then preferably connected to a source of alternating current and immersed in the electrolyte or bath. They may, however, also be first immersed in the bath whereupon the current is switched on quickly. If desired the liquid in the bath may be agitated during the treatment. If an acid bath is employed its action is generally increased by raising the temperature of the liquid. The process may be carried through at ordinary room temperatures but preferably the temperature of the bath is kept at 40 to 50 C. or higher. The bath may be provided with a cooling device in order to regulate the temperature in case of excessive heating by the electric current.

The composition and concentration of the acid liquor or electrolyte in the bath may, according to the eflect to be obtained and according to the composition and manner of preparation of the metal, be varied within wide limits. Concentrated solutions act considerably more powerfully and require correspondingly shorter periods of treatment to obtain the same effect than diluted solutions. The voltages or currents may likewise vary within wide limits according to the result aimed at and according to the composition of the aluminium alloys to be treated and the electrolyte employed as well as its temperature.

After the treatment with alternating current in the acid bath, the articles are lifted out of the vat while under potential or shortly after the current has been switched off, rinsed with water and dried.

Examples (1) Two sheets of pure aluminium are connected to a supply of alternating current of 40 volts, immersed in a 25% aqueous solution of chromic acid heated to 60" C., and allowed to remain under current for 30 minutes whereupon the current is switched off. The aluminium sheets are then quickly removed from the bath, rinsed and dried. The surface of the sheets then have a dark blue-black color.

2) Two duralumin sheets are placed into a aqueous sulphuric acid solution through which an alternating current of 10 to volts is sent during minutes. After switching oil the current and removing the sheets from the bath they have a silverwhite color. They are rinsed with water and dried.

(3) Two Lautal aluminium sheets are treated with an alternating current of 40 volts in a bath of aqueous chromic acid heated to 57 C. for 20 minutes. After rinsing and drying the sheets have a grey color.

4) Two plates of duralumin are treated in an aqueous solution which contains 10% permanganate of potassium and 10% chromic acid with alternating current of volts at C. for about 20 minutes. After rinsing and drying the sheets have a brown color.

(5) Two plates of die cast aluminium alloys with 70.9% aluminium, 4% copper, 2.7% nickel and 2.6% silicon are treated during 10 minutes in a bath of 25% chromic acid heated to 49 'C. with alternating current of '7 to 7.5 volts. The surface of the metal has after rinsing and drying a mother of pearl lustre, iridizing from green to violet-red.

6 Two strips A of duralumin are dipped into a bath at ordinary temperatures containing 0.05% of chromic acid and connected to a source of alternating current. The voltage is gradually raised from a low value at the start. without arcing occurring, to 100 volts and more and the strips are allowed to remain under current for After the removal of to a source of alternating current, the voltage being gradually raised from 5 volts to 40 or 50 volts. After one hours treatment the sheet B is covered with a deep black protective coating 5 while the sheet A is unchanged. After removal of the sheet B from the bath any desired number of untreated sheets may be substituted for the sheet B and treated in a similar manner. The appearance of the coatings of the sheets B is always the same, apart from slight differences in the shades which are traceable to differences in the composition of the alloys and in the condition of the surface and the structure-of the metal. (7) Two strips of duralumin are connected to a source of alternating current in a bath containing 10% of chromic acid heated to 40 C. The voltage is gradually raised to 40 to 50 volts and. the treatment is continued for about to minutes. One sheet A is bright, the other sheet B is covered with a pearlgrey coating. In the place of the sheet B a fresh untreated sheet B1 is now mounted opposite the sheet A and treated in the same manner with an alternating current gradually rising to or 50 volts during one hour. The sheet B1 is then black while sheet A is unchanged. In place of sheet B1 any desired number of sheets B may be treated in a similar manner. The protective coatings on the sheets B are uniformly black with the exception of slight variations in the shade.

(8) An aluminium sheet A brightly coated in diluted chromic acid is treated with alternating current opposite an untreated aluminium sheet B in an aqueous 50% chromic acid solution heated to 40 C. in such a manner that, starting with low voltages, the voltage is gradually raised to 40 to 50 volts. After one hours treatment the sheet B is deep black. This deep black coating, which is obtained from a solution containing at 40 least about 40% chromic acid, is more flexible than the coating obtained according to Example 1, it is less inclined to crack so that it is, for example, very suitable as insulating layer for aluminium coils.

(9) Two articles of Lautal (aluminium from the Lauta Works) one of which is brightly coated in a diluted chromic acid solution, while the other is untreated, are connected to the terminals of an alternating current supply and subjected to the action of an alternating current in a bath heated to 40 C. of aqueous, 3.5%

chromic acid.

The treatment is started at a voltage of about 10 volts and the voltage is gradually raised to 40 volts within the first 10 minutes, within a further 10 minutes to volts, and finally again in 10 minutes to volts and then maintained at 50 Volts during 30 minutes. The untreated 1 article is then covered with a grey-black protective coating.

(10) Two Lautal sheets connected to the terminals of an alternating current supply, one of which is dark coated, the other untreated, are treated with alternating current in a 4% aqueous solution of chromic acid heated to 60 C. the voltage'being gradually increased from 0 to 40 volts within one hour. The originally dark coated sheet then becomes brighter while the fresh sheet treated opposite to it appears black. The coated sheet which has become brighter may be used as counter-electrode any number of times for producing the dark coatings on fresh Lautal sheets.

(11) A sheet of pure aluminium and an article of duralumin are treated in a 2% bath of chromic acid heated to 50 C. during 1 hour at gradually increasing voltages up to 40 volts. The article of duralumin is then covered with a dark protective coating while the sheet of aluminium remains bright. Opposite to this sheet of aluminium any desired number of articles of duralumin or Lautal may be coated with dark protective coatings by a'similar treatment with alternating current.

(12) In a 1% solution of potassium hydroxide at room temperature (20 C) two Lauta sheets were connected to the terminals of an alternating current supply and treated with alternating current during 30 minutes, the voltage being gradually raised from 0 to 20 volts. Both sheets were covered with greyish-black protective coatings after this treatment which provided an efflcient protection against corrosion.

(13) Two sheets of pure aluminium were treated with alternating current in a bath of a 1% potassium hydroxide solution at ordinary temperatures during about 40 minutes, the temperature being gradually raised to 49 C. Fairly smooth pearl-grey and corrosion-proof protecting coatings were produced on both sheets.

(14) In a solution of carbonate of calcium in aqueous ammonia two aluminium electrodes were treated with alternating current during one hour the voltage being gradually raised from 0 to 20 volts. In this case light grey protective coatings resulted on both electrodes which were again well able to resist corrosion.

The various protective coatings produced according to the Examples 1 to 5 showed throughout a satisfactory resistance against corrosive action as was proved by continued tests in a salt water atmosphere produced by spraying or atomization. The peculiar phenomenon was, however, observed that the protective coatings on the two electrodes, even when using electrodes of the same material, frequently showed difierent colors. While one electrode was, for instance, covered with a light-grey or dark-grey coating, the coating of the second electrode remained frequently light or I even whitish. These peculiar differences in color were particularly frequent in alloys of the nature of duralumim or Lautal while they developed less frequently on pure aluminium.

In case of the Examples 6 to 11, on the other 2 hand, protective coatings of uniform color in bright or dark shades were obtained which in regard to their ability of resisting corrosion were at least equal or even superior 'to the protective coating produced according to the Examples 1 to 5. It was ascertained that uniformly colored protective coatings could always be obtained if an article provided with a light or dark protective coating, which may have been produced according to one of the Examples 1 to 5, is used as often as desired as a counter-electrode for the production of protective coatings on other articles constructed of the same or of another light metal or alloy. A brightly colored coating may, for instance, be precipitated previously on the article serving as a counter-electrode from a chromic acid solution diluted as far as possible and the uniformly colored protective coating may be produced on other articles by using the original aqueous chromic acid solution. In some cases the production of the protective coating on the counter electrode is preferably carried through in an aqueous solution of chromic acid of considerably lower concentration than that ,ess may also be carried through with alkaline baths, as shown in the Examples 12 to I4. In this case also very efiicient anticorrosive coatings are obtained. Besides the baths stated in the examples aqueous alkaline solutions of manganates,

molybdates, antimonates, titanates, wolframates,

niobates and similar substances may be used for the preparation of the electrolytes or as additions thereto.

The articles treated according to our improved process are preferably covered with further protective coatings or films in order to increase their resistance to corrosion still further, such as films of grease, wax or their mixtures, and coats of oil paint, tar, asphalt and similar anticorrosion agents well known in the art.

Our improved process is, of course, not limited to the conditions stated in the examples and it is left to the operator to vary the quality and color of the protective coatings by changing the concentration of the bath, its temperature, the duration of the treatment and the voltage of the current applied.

It is, furthermore, immaterial whether in carrying out our improved process, the two or more electrodes consist of the same light metal or light metal alloy or of different light metals or light metal alloys. In all cases useful protective coatings can be produced by a proper choice of the electrolytes and by suitable operating conditions.

Preferably a sinusoidal alternating" current should be used for our improved process. It is, however, possible to employ an alternating current the current curve of which deviates from the sinus form or an alternating current on which a direct current has been superimposed. In the examples of the process stated herein an alternating current of a frequency of 50 cycles per second has generally been used. Our improved process may. however, equally well be carried through with an alternating current, the frequency of which is less or more than 50 cycles per second. The current density may in our improved process lie between 0.4 to 1.5 amperes per square decimetre.

We claim as our invention:

1. In the production of uniformly colored protective coatings on articles of aluminum and aluminum alloys, the process which comprises passing alternating current through an electrolytic bath between such anarticle and an electrode of similar metal, the said electrode having been previously provided with an oxidecontaining coating in a prior operation.

2. The process of claim 1 in which the electrolytic bath contains chromic acid.

3. The process of claim 1 in which the said electrode has been previously coated in a dilute chromic acid solution.

4. In the production of uniformly colored pro- .tective coatings on metal articles of aluminum and aluminum alloys, the process which comprises producing a bright oxide-containing coating on an electrode of such a metal in a dilute chromic acid bath and passing alternating current between said electrode and an article to be coated in a second bath of chromic acid of 'a higher concentration than said first bath.

5 In the production of uniformly colored protective coatings on articles of aluminum and aluminum alloys, the process which comprises coating such an article by passing alternating current through an electrolytic bath, between the same and an oxide coated electrode, rinsing and drying the coated article and finally covering the article with a protective film of wax, asphalt, paint or equivalent.

6. In the production of protective coatings on metal articles of aluminum and aluminum alloys, the process which comprises passing an electric current at a potential of about 40 volts between such an article and a counter electrode of such a metal previously provided with an oxide-containing coating, in an electrolytic bath containing about 25 per cent of chromic acid and maintained at a temperature in the neighborhood of 50 C.

7. In the process of producing protective electrolytic coatings on metal articles of aluminum and aluminum alloys by passing alternating current between articles of such metals in an electrolytic bath containing chromic acid, the improvement which comprises employing a counter electrode of such a metal which has been previously coated with an oxide-containing coating, whereby a dark colored protective coating is deposited on said article.

ALEXANDER JENNY. HEINZ SIEBENEICHER. NICOLAI BUDILOFF.

CERTiFIC-ATE or CORRECTION.

Patent No. 1,923,539. August 22, 1933.

AILIEIQANDER JENNY, ET AL.

it is hereby certified that the name of the assignee in the above numbered patent was erroneously described and specified as "Siemens-Eiectro-Asmose G. M. B. H." whereas said name should have been described and specified as "Siemens-Eiectro-Osmose G. M. B. H." as shown by the records of assignments in this office; and that the said Letters Patent shouid be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 26th day of September, A. D. 1933.-

h. M. Hnplkins (Seal) Acting Commissioner oi Patents, 

